1. Field of the Invention
Embodiments disclosed herein generally relate to drill bits for drilling earth formations. In particulars, embodiments disclosed herein relate to cutters for a fixed cutter drill bit.
2. Background Art
Rotary drill bits with no moving elements on them are typically referred to as “drag” bits or fixed cutter drill bits. Drag bits are often used to drill a variety of rock formations. Drag bits include those having cutters (sometimes referred to as cutter elements, cutting elements, polycrystalline diamond compact (“PDC”) cutters, or inserts) attached to the bit body. The cutters may be formed having a substrate or support stud made of carbide, for example tungsten carbide, and an ultrahard cutting surface layer or “table” made of a polycrystalline diamond or polycrystalline boron nitride material deposited onto or otherwise bonded to the substrate at an interface surface.
An example of a prior art drag bit having a plurality of cutters with ultrahard working surfaces is shown in FIG. 1. The drill bit 10 includes a bit body 12 and a plurality of blades 14 that are formed on the bit body 12. The blades 14 are separated by channels or gaps 16 that enable drilling fluid to flow between to clean and cool the blades 14 and cutters 18. Cutters 18 are held in the blades 14 at predetermined angular orientations and radial locations to present working surfaces 20 with a desired back rake angle against a formation to be drilled. The working surfaces 20 are generally perpendicular to the axis 19 and side surface 21 of the cylindrical cutter 18. Thus, the working surface 20 and the side surface 21 meet or intersect to form a circumferential cutting edge 22.
Nozzles 23 are typically formed in the drill bit body 12 and positioned in the gaps 16 so that fluid can be pumped to discharge drilling fluid in selected directions and at selected rates of flow between the blades 14 for lubricating and cooling the drill bit 10, the blades 14, and the cutters 18. The drilling fluid also cleans and removes cuttings as the drill bit 12 rotates and penetrates the geological formation. The gaps 16, which may be referred to as “fluid courses,” are positioned to provide additional flow channels for drilling fluid and to provide a passage for cuttings to travel past the drill bit 10 toward the surface of a wellbore (not shown).
The drill bit 10 includes a shank 24 and a crown 26. Shank 24 is typically formed of steel or a matrix material and includes a threaded pin 28 for attachment to a drill string. Crown 26 has a cutting face 30 and outer side surface 32. The particular materials used to form drill bit bodies are selected to provide adequate toughness, while providing good resistance to abrasive and erosive wear. For example, in the case where an ultrahard cutter is to be used, the bit body 12 may be made from powdered tungsten carbide (WC) infiltrated with a binder alloy within a suitable mold form. In one manufacturing process the crown 26 includes a plurality of holes or pockets 34 that are sized and shaped to receive a corresponding plurality of cutters 18.
The combined plurality of surfaces 20 of the cutters 18 effectively forms the cutting face of the drill bit 10. Once the crown 26 is formed, the cutters 18 are positioned in the pockets 34 and affixed by any suitable method, such as brazing, adhesive, mechanical means such as interference fit, or the like. The design depicted provides the pockets 34 inclined with respect to the surface of the crown 26. The pockets 34 are inclined such that cutters 18 are oriented with the working face 20 at a desired rake angle in the direction of rotation of the bit 10, so as to enhance cutting. It will be understood that in an alternative construction (not shown), the cutters can each be substantially perpendicular to the surface of the crown, while an ultrahard surface is affixed to a substrate at an angle on a cutter body or a stud so that a desired rake angle is achieved at the working surface.
A typical cutter 18 is shown in FIG. 2. The typical cutter 18 has a cylindrical cemented carbide substrate body 38 having an end face or upper surface 54 referred to herein as the “interface surface” 54. An ultrahard material layer (cutting layer) 44, such as polycrystalline diamond or polycrystalline cubic boron nitride, forms the working surface 20 and the cutting edge 22. A bottom surface 52 of the ultrahard material layer 44 is bonded on to the upper surface 54 of the substrate 38. The bottom surface 52 and the upper surface 54 are herein collectively referred to as the interface 46. The top exposed surface or working surface 20 of the cutting layer 44 is opposite the bottom surface 52. The cutting layer 44 typically has a flat or planar working surface 20, but may also have a convex exposed surface, that meets the side surface 21 at a cutting edge 22.
Cutters may be made, for example, according to the teachings of U.S. Pat. No. 3,745,623, whereby a relatively small volume of ultrahard particles such as polycrystalline diamond or cubic boron nitride is sintered as a thin layer onto a cemented tungsten carbide substrate. Flat top surface cutters, as shown in FIG. 2, are generally the most common and convenient to manufacture with an ultrahard layer, according to known techniques. It has been found that cutter chipping, spalling, and delamination are common failure modes for ultrahard flat top surface cutters.
Generally speaking, the process for making a cutter 18 employs a body of tungsten carbide as the substrate 38. The carbide body is placed adjacent to a layer of ultrahard material particles such as polycrystalline diamond or cubic boron nitride particles and the combination is subjected to high temperature at a pressure where the ultrahard material particles are thermodynamically stable. This results in recrystallization and formation of a polycrystalline ultrahard material layer, such as a polycrystalline diamond or polycrystalline cubic boron nitride layer, directly onto the upper surface 54 of the cemented tungsten carbide substrate 38.
Different types of bits are generally selected based on the nature of the geological formation to be drilled. Drag bits are typically selected for relatively soft formations such as sands, clays and some soft rock formations that are not excessively hard or excessively abrasive. However, selecting the best bit is not always straightforward, because many formations have mixed characteristics (i.e., the geological formation may include both hard and soft zones), depending on the location and depth of the well bore. Changes in the geological formation can affect the desired type of bit, the desired rate of penetration (ROP) of a bit, the desired rotation speed, and the desired downward force or weight-on-bit (“WOB”). Where a drill bit is operated outside the desired ranges of operation, the bit can be damaged or the life of the bit can be severely reduced.
For example, a drill bit normally operated in one general type of formation may penetrate into a different formation too rapidly or too slowly subjecting it to too little load or too much load. In another example, a drill bit rotating and penetrating at a desired speed may encounter an unexpectedly hard formation, possibly subjecting the bit to a sudden impact force. A formation material that is softer than expected may result in a high rate of rotation, a high ROP, or both, thereby causing the cutters to shear too deeply or to gouge into the geological formation.
Such conditions may place greater loading, excessive shear forces, and added heat on the working surface of the cutters. Rotation speeds that are too high without sufficient WOB, for a particular drill bit design in a given formation, can also result in detrimental instability (bit whirling) and chattering because the drill bit cuts too deeply or intermittently bites into the geological formation. Cutter chipping, spalling, and delamination, in these and other situations, are common failure modes for ultrahard flat top surface cutters.
Dome top cutters, which have dome-shaped top surfaces, have provided certain benefits against gouging and the resultant excessive impact loading and instability. This approach for reducing adverse effects of flat surface cutters is described in U.S. Pat. No. 5,332,051. An example of such a dome cutter in operation is depicted in FIG. 3. The prior art cutter 60 has a dome-shaped top or working surface 62 that is formed with an ultrahard layer 64 bonded to a substrate 66. The substrate 66 is bonded to a metallic stud 68. The cutter 60 is held in a blade 70 of a drill bit 72 (shown in partial section) and engaged with a geological formation 74 (also shown in partial section) in a cutting operation. The dome-shaped working surface 62 effectively modifies the rake angle A produced by the orientation of the cutter 60.
Scoop top cutters, as shown in U.S. Pat. No. 6,550,556, have also provided some benefits against the adverse effects of impact loading. This type of prior art cutter is made with a small “scoop” or depression formed on a substrate and an ultrahard layer, wherein the depression extends radially outward to a substrate periphery. The ultrahard layer is bonded to a substrate at an interface. The depression is formed in the critical region, such that the scooped or depressed region is in contact with the formation.
Beveled or radiused cutters have provided increased durability for rock drilling. U.S. Pat. Nos. 6,003,623 and 5,706,906 disclose cutters with radiused or beveled side walls. This type of prior art cutter has a cylindrical mount section with a cutting section, or diamond cap, formed at one of its axial ends. The diamond cap includes a cylindrical wall section. An annular, arc surface (radiused surface) extends laterally and longitudinally between a planar end surface and the external surface of the cylindrical wall section. The radiused surface is in the form of a surface of revolution of an arc line segment that is concave relative to the axis of revolution.
While conventional PDC cutters have been designed to increase the durability for rock drilling, cutting efficiency usually decreases. The cutting efficiency decreases as a result of the cutter dulling, thereby increasing the weight-bearing area. As a result, more WOB must be applied. The additional WOB generates more friction and heat and may result in spalling or cracking of the cutter. Additionally, ROP of the cutter may be decreased. Further, sudden high advance rates are common as the cutters tend to slide over the formation without engaging the formation. Balling of the formation is also a common concern in drilling in soft information.
Accordingly, there exists a need for a cutting structure for a PDC drill bit that more efficiently removes formation.